1. Field of the Invention
The present invention relates to a ceramic slurry composition for producing a thin green sheet using an extrusion-stretching process, and an electronic device fabricated using the green sheet. More particularly, the present invention relates to a ceramic slurry composition for producing a thin green sheet which has a thickness of 10 μm or less and can be laminated to form a 40-layer or more stack, a method for producing the green sheet by extrusion, and an electronic device fabricated using the green sheet.
2. Description of the Related Art
Generally, chip components such as MLCCs (Mutilayer Ceramic Capacitors), chip inductors and the like are fabricated by alternately laminating ceramic green sheets and internal electrodes, and pressing the laminated structure and sintering it.
Ceramic green sheets are commonly produced by a tape casting process. According to the tape casting process, the green sheets are produced by coating a ceramic slurry solution with a uniformly dispersed ceramic powder therein onto a support film using a die coater, doctor blade, etc., and drying the coated slurry.
To produce a few tens of micrometers or less thick thin green sheet using the tape casting process, the ceramic slurry solution should have a low viscosity. When the viscosity of the slurry solution is, however, too low, the shape maintenance of a film coated onto the support film becomes difficult, causing an inhomogeneous thickness of a final sheet. In addition, due to a poor strength of the green sheet, damages to the green sheet, such as splitting, are likely to occur when the green sheet is removed from a substrate. Furthermore, lamination of the thin green sheet is difficult.
In order to prevent the damages to the green sheet, polymers used as binders must have a high molecular weight. However, when the molecular weight of the polymers is too high, it is difficult to produce a green sheet in the form of a thin film. In addition, since the solubility of the polymers in solvents is low, the polymer particles are precipitated in an inhomogeneously gelled state, which leads to defects in chip components fabricated using the high molecular weight polymers as binders.
PCT publication WO 91/01346 suggests a green sheet which can be produced using an extrusion-stretching process. According to the prior art, a green sheet having a thickness of 25 μm or less is produced by extrusion-stretching slurry including a ceramic powder, a polymer having a molecular weight of 400,000 or more, and a solvent for gelling the polymer. Since the extrusion-stretching process does not require any substrate to produce the green sheet, the green sheet has a uniform thickness, compared to green sheets produced using a tape casting process.
In the prior art, since the green sheet is produced using the ultra-high molecular weight polymer, there is a problem of a poor interlayer adhesive strength when laminated and pressed, due to a high modulus of elasticity and a non-polarity of the polymer. Accordingly, when the green sheet is laminated and sintered, cracks and delamination occur between the laminated green sheets and thus it is difficult to fabricate chip components using the green sheets.
As recent technical development in the field of compact and high performance electronic devices has progressed, there exists a need for small chip components (electronic devices). To meet this need, thin green sheets are required to have a thickness of 10 μm or less and be laminated to form a 40-layer or more stack. However, since the green sheet of the prior art has a problem that the cracks between layers occur due to its poor interlayer adhesive strength, it cannot be utilized in the fabrication of 40 layers or higher chip components. The lamination of green sheets into 40 layers or higher for fabricating high performance electronic devices causes a problem of ‘pillowing’. This pillowing refers to a phenomenon that when green sheets and internal electrodes are laminated to each other and pressed, a margin zone where no internal electrodes are provided or sufficient internal electrodes are not provided is lower in height than an active zone where sufficient internal electrodes are provided, and thus the margin zone is collapsed and the active zone is swelled (FIG. 3a). This pillowing phenomenon results in deformation of chip appearance.